The present invention relates to a magnetically operated drain valve of an electrohydraulic lifting module, in particular for stacker trucks, comprising a closing element which in a closing direction is assigned to a main valve seat and which in the opening direction is actuatable by the drain pressure and in the closing direction by a variable difference between the drain pressure and a control pressure derived from the load pressure.
In small, inexpensive stacker trucks, a concept of the lifting module is known in practice, according to which the drain control of the pressurized fluid is performed by a black/white drain valve, as is known from the data sheet D 7490/1 of the company Heilmeier & Weinlein, 81673 Munchen, which was printed in March 1996. Two different connecting modes of the drain valve are possible. In the first instance the drain valve shuts off fluid in the currentless state of the magnet. The lifting movement is controlled via the pump. The lowering movement is controlled by means of the drain valve which is moved into the passage position by the magnet which has current applied thereto. The pilot valve is fully opened for the lowering function, so that in the relieved state of the control chamber the load pressure lifts the closing element in opening direction over the whole opening stroke via a differential surface on the closing element. In the second mode the drain valve is held in the passage position in the currentless state of the magnet. Supplementary, a two-position switching valve is provided between the consumer and the branch towards the drain valve. The lifting and lowering movements are controlled by means of two valves. During the lifting operation the drain valve is moved into the closing position by the application of current to the magnet, with the two-position switching valve being in the load holding position, and the lifting speed is controlled by the pump. The drain valve is set to the closing position and the two-position switching valve to the drain position for lowering purposes before the drain valve is switched to the passage position. Independently of the connection mode, no ramp function can be controlled with the black/white drain valve during lowering, the ramp function being desired for the stacker truck. The spring which in the first mode is provided as a dosing spring and in the second mode as an opening spring for the pilot valve is soft in both cases, i.e., it has a spring characteristic without any considerable rising gradient. Furthermore, the areas of the nozzle and the passage in the pilot valve are as large as possible in order to obtain a rapid response of the drain valve and are, for instance, designed with respect to the minimun drain amount. The closing element cooperates exclusively with a seat function with the main valve seat to ensure absolute tightness in the closing position of the drain valve. Such tightness is required to make sure that the load pressure is maintained even over a long time.
In more complicated lifting modules for large-sized and expensive stacker trucks, a connection principle according to DE-C2-42 39 321 is known in which the drain control is performed via a two-way flow controller which is given a "truck-tight" operating behaviour (extremely small leakage in closed position, only allowing e.g. a motion of a load for 1 cm/hour). Although there is a ramp function during lowering movements, the constructional efforts required therefor are considerable, so that this lifting module is not used in small and inexpensive stacker trucks for reasons of costs.